The present invention relates in general to a toroidal-type continuously variable transmission used in vehicles such as automobiles, and more particularly to a power roller of a rolling element used for the transmission, and a method for producing the power roller. This rolling element of the transmission is defined as being a combination of the power roller, an input side disk and an output disk, as will be clarified hereinafter.
A conventional toroidal-type continuously variable transmission is, as shown, for example, in FIG. 6, constructed so that an input side disk 11 and an output side disk 12 are coaxially disposed so as to be opposed to each other inside a housing (not shown in FIG. 6). An input shaft 13 passes through the shaft center of the toroidal transmission section having the input side disk and the output side disk. A loading cam 14 is disposed on an end of the input shaft 13. The loading cam 14 transmits the motive power (rotational force) of the input shaft 13 to the input side disk 11 through a cam roller 15.
The input side disk 11 and the output side disk 12, having substantially the same shape, are disposed so as to be symmetrical, and formed so as to be substantially semicircular in section as viewed in the axial direction with both opposed surfaces thereof taken into view. A pair of power rollers 16 and 17 that transmit motion are disposed so as to be in contact with the input side disk 11 and the output side disk 12, respectively, within a toroidal cavity formed by the toroidal surfaces of the input side disk 11 and the output side disk 12. Reference numeral 23 designates thrust ball bearings. In this case, the power rollers 16 and 17 are pivotably attached to trunnions 20 and 21 through pivots 18 and 19, and pivotably supported with a pivot A as the center, the pivot A serving as the center of the toroidal surface of the input side disk 11 and the output side disk 12. The surfaces of contact among the input side disk 11, the output side disk 12, and the power rollers 16 and 17 are supplied with a lubricating oil whose viscous frictional resistance is large, so that the motive power applied to the input side disk 11 is transmitted to the output side disk 12 through the lubricating oil film and the power rollers 16 and 17.
The input side disk 11 and the output side disk 12 are independent of the input shaft 13 (not being directly affected by the motive power of the input shaft 13) through needles 25. An output shaft 24 is attached to the output side disk 12. The output shaft extends in parallel with the input shaft 13 and is rotatably supported by the housing through an angular bearing 22.
In this toroidal-type continuously variable transmission, the motive power of the input shaft 13 is transmitted to the loading cam 14. When the loading cam 14 is rotated by the transmission of the motive power, this rotational power is transmitted to the input side disk 11 through the cam roller 15, which in turn causes the input side disk 11 to rotate. The motive power generated by the rotation of the input side disk 11 is transmitted to the output side disk 12 through the power rollers 16 and 17. The output side disk 12 rotates integrally with the output shaft 24.
At the time of changing the speed, the two trunnions 20 and 21 are slightly moved toward the pivot A. That is, the axial movement of the trunnions 20 and 21 releases the intersection between the rotating shaft of the power rollers 16 and 17 and the shafts of the input side disk 11 and the output side disk 12. As a result, the power rollers 16 and 17 oscillates over the surfaces of the input side disk 11 and the output side disk 12, thereby changing the speed ratio to either accelerate or decelerate the motor vehicle.
Such a toroidal-type continuously variable transmission is disclosed, for example, in U.S. Pat. No. 5,556,348 corresponding to Japanese Patent Unexamined Publication No. 7-71555. In this patent '348, effective carburized depths of the input side disk, the output side disk, and the power roller are limited to fall in a range of from 2.0 mm to 4.0 mm. As conventional examples of the above-mentioned input side disk, output side disk, and power rollers, those using AISI 52100 (equivalent of a high carbon chromium bearing steel having a symbol of SUJ 2 according to Japanese Industrial Standard (JIS) G 4805, 1970) are known (see NASA Technical Note, NASA TN D-8362, Dec. 1976).
When the above-mentioned toroidal-type continuously variable transmission is driven, the bearing surface of the power roller receives a high load from the input side disk and the output side disk. Furthermore, the power roller rotates at a high speed, while engaging with both the input side disk and the output side disk. Thus, the bearing surface of the power roller is subjected to high temperature and high pressure, and therefore tends to flake due to the rolling contact fatigue. In other words, when the power roller is in a rolling contact with the input side disk and the output side disk under high temperature and high pressure, heat generated by the rolling contact tends to lower the hardness of an outer layer of the power roller. With this, the outer layer lowers in fatigue strength and thus tends to flake. In view of this, the outer layer of the power roller is required to have a sufficient strength or hardness at high temperature, that is, temper hardness. The power roller may be subjected to a hardening process, that is, a carburizing or carbonitriding process, in order to harden the surface of the same. The strength of the power roller at high temperature is greatly affected by the carbon or carbon and nitrogen concentrations of the outer layer of the same which has been subjected to the hardening process. In fact, when the outer layer of the power roller is low in carbon or carbon and nitrogen concentrations, this outer layer becomes low in high-temperature strength. With this, the power roller becomes insufficient in rolling fatigue life. Furthermore, if each outer layer of the power rollers varies to a great extent in carbon or carbon and nitrogen concentrations, the outer layer will have a wide variation in high-temperature strength and thus in rolling fatigue life. This is a disadvantage in the production of power roller in an industrial scale.